The aim of this investigation was to correlate the development of the endocochlear potential with the morphological postnatal development of the stria vascularis. Kittens 1 to 30 days old were used. The physiological observations demonstrated that the magnitude of the endocochlear potential was a linear function of age from 1 to 27 days. For the ultrastructural studies the ear was perfused with 2.67% osmium tetroxide. These studies showed a striking difference in morphology between the cells of the stria in relation to age. In one day old cats, marginal and intermediate cells show few compartmentation. The basal cells are large elements with clear cytoplasm. The compartmentation of marginal and intermediate cells increase in number with age. The size of basal cells decrease and its cytoplasm becomes dense. The strial cells in 30 day old animals are morphologically similar to the adult cat. Apparently, there is astructural basis in the stria vascularis for the characteristic postnatal development of endocochlear potential.